Solar distillation differs from other forms of desalination that are more energy-intensive, such as methods such as [[reverse osmosis]], or simply boiling water due to its use of free energy.<ref> Abu-Arabi, M. (2007). Status and prospects for solar desalination in the MENA region. Solar Desalination for the 21 st Century, 163-178.</ref><ref>

Paton, C., & Davies, P. (2006). The seawater greenhouse cooling, fresh water and fresh produce from seawater. In The 2nd International Conference on Water Resources in Arid Environments, Riyadh.</ref> A very common and, by far, the largest example of solar distillation is the natural water cycle that the Earth experiences.

</ref> [[File:Screen Shot 2012-12-14 at 11.17.07 PM.png|thumb|right|Carlos Wilson, Swedish Engineer<ref name="Emmy"/>]]This desalination plant,"can be considered to be the first industrial installation for exploitation of solar energy<ref name="Hirsch"/>." This desalination The Las Salinas plant was envisioned to take advantage of the nearby saltpeter mining effluent to supply the miners and their families freshwater <ref name="Emmy"/>.The facility was quite large for its time and now:<blockquote>"The plant was constructed of wood and timber framework covered with one sheet of glass. It consisted of 64 bays having a total surface area of 4450 m2 m<sup>2</sup> and a total land surface area of 7896 m2m<sup>2</sup>. It produced 22.70 m3 m<sup>3</sup> of fresh water per day. The plant was in operation for about 40 years until the mines were exhausted<ref name="Emmy"/>."

</blockquote>

Interest in solar distillation wavered for some time, until historical events prompted further research and development. World War II was a great catalyst for the Massachusetts Institute of Technology to develop appropriate solar stills for use in more remote areas of the world during emergencies. These small solar stills were made to float on and collect saltwater to desalt as they floated alongside life-boats and rafts<ref name="Emmy"/>. More siginificant studies into solar distillation were carried out by the Office of Saline Water, a sector the US government, in 1952. Many experiments were performed on different conceptualizations of the solar still, including multiple-effect basins and the application of condensers<ref name="Emmy"/>. This trend ended near the early 70's with the advent of more lucrative desalination techniques like the aforementioned reverse osmosis or multi-stage flash, a technique that involves a series of stages where evaporation relies on lowering the pressure of each stage to lower the boiling or "flashing" point of the water <ref> El-Dessouky, H. T., Ettouney, H. M., & Al-Roumi, Y. (1999). Multi-stage flash desalination: present and future outlook. Chemical Engineering Journal, 73(2), 173-190. </ref><ref name= "Fath">

Interest in solar distillation wavered for some time, until historical events prompted further research and development. World War II was a great catalyst for the Massachusetts Institute of Technology to develop appropriate solar stills for use in more remote areas of the world during emergencies. These small solar stills were made to float on and collect saltwater to desalt as they floated alongside life-boats and rafts<ref name="Emmy"/>. More significant studies into solar distillation were carried out by the Office of Saline Water, a sector the US government, in 1952. Many experiments were performed on different conceptualizations of the solar still, including multiple-effect basins and the application of condensers<ref name="Emmy"/>. This trend ended near the early 70's with the advent of more lucrative desalination techniques like the aforementioned reverse osmosis or multi-stage flash, a technique that involves a series of stages where evaporation relies on lowering the pressure of each stage to lower the boiling or "flashing" point of the water <ref> El-Dessouky, H. T., Ettouney, H. M., & Al-Roumi, Y. (1999). Multi-stage flash desalination: present and future outlook. Chemical Engineering Journal, 73(2), 173-190. </ref><ref name= "Fath">

The fundamental aspects of a solar still have gone unchanged since ancient times, the simplicity of the design is one of the solar still’s chief benefits. However, there are many variations on the theme of the typical single slope/basin still and these can fall into one of two categories, active or passive. These labels classify the still by the method it uses to acquire the energy to drive the evaporation of the water. Passive solar stills are, of course, more conventional and have been the only ones discussed up to this point. [[File:Solar Still Flow Chart.jpg|thumb|left|Types of Solar Stills<ref name = "Goosen"/>]] Active stills, however, can obtain "waste" heat from a myriad of sources.

The fundamental aspects of a solar still have gone unchanged since ancient times, the simplicity of the design is one of the solar still’s chief benefits. However, there are many variations on the theme of the typical single slope/basin still and these can fall into one of two categories, active or passive. These labels classify the still by the method it uses to acquire the energy to drive the evaporation of the water. Passive solar stills are, of course, more conventional and have been the only ones discussed up to this point. Active stills, however, can obtain "waste" heat from a myriad of sources. A good insulator is necessary to reduce thermal losses and prolong the evaporation process even into the night<ref>

Löf, G. O. (1961). Fundamental problems in solar distillation. Proceedings of the National Academy of Sciences of the United States of America, 47(8), 1279.

The main design challenge is keeping the distiller airtight. If not airtight, efficiency drops severely <ref name ="Gordes"/>.

Often a shallow trough is used, painted black, and flooded. A slanted pane of glass covering, allowing condensed water vapor to slide down into an output channel. Expect 1 gallon per day per square meter of glass.'''Passive Solar Stills'''

Conventional solar stills rely solely on the sun to distill water, however their complexity could still reach that of active stills, if not other more intricate desalination methods. Passive stills, then, vary widely due to this one constraint and can be further organized into sub-classes. Some common types of passive solar stills include<ref name ="Fath"/>:*Single-effect*Multi-effect*Basin*Double Slope*Wick*Multi-wick*Diffusion*Greenhouse'''''Single-effect stills''''' are the simplest and most common, since only one interface is necessary to convey the energy and collect the condensate. An example of a crucial design challenge in all solar stills is keeping the distiller airtight. If not airtight, efficiency drops severely <ref name ="Gordes"/>.Often a shallow trough is used, painted black, and flooded. A slanted pane of glass covering, allowing condensed water vapor to slide down into an output channel. Expect 1 gallon per day per square meter of glass. Another approach is molded [[plastic]], e.g. the Watercone (see [[#Watercone|below]]). This has the advantage that it is can be more easily made airtight, and mass production should make it affordable. '''''Multi-effect stills''''' require double the effort in regards to ensuring tight seals, and could be more to difficult to clean, but they can raise the production of freshwater significantly <ref>Blanco, J., & Alarcón, D. (2007). The PSA experience on solar desalination: technology development and research activities. Solar Desalination for the 21 st Century, 195-206.</ref>. The way, by which, the water is stored for its time in the liquid phase can also contrast. '''''Basin-type stills''''' contain the water in an impervious material that is a component of the entire enclosure and are the most ubiqutious. '''''Wick stills''''' use cloth-like materials that use capillary action to propagate the water through the system. When efficiency and effectiveness are key, wick stills out-produce basin stills due to the greater surface area of evaporation, lower energy cost to heat the water, and ability to create a much larger effective area for solar radiation to transfer energy into the water<ref>Noble, Neil (2012). Solar Distillation. Retrieved from http://http://practicalaction.org/solar-distillation-1</ref>.'''''Multi-wick stills''''' obviously play off of typical wick stills and much like the multi-effect premise from above, they greatly increase the productivity for increasing the influenced surface area exponentially<ref>Velmurugan, V., & Srithar, K. (2011). Performance analysis of solar stills based on various factors affecting the productivity—A review. Renewable and Sustainable Energy Reviews, 15(2), 1294-1304.</ref>. '''''Diffusion-type stills''''' run with the ideas introduced by the mutli-effect & -wick stills and a further advancement to both. Perhaps, Tanaka & Nakatake best explain the design behind these efficient stills, "which consist of a series of closely spaced parallel partitions in contact with saline-soaked wicks, have great potential because of their high productivity and simplicity<ref>Tanaka, H., & Nakatake, Y. (2007). Outdoor experiments of a vertical diffusion solar still coupled with a flat plate reflector. Desalination, 214(1), 70-82.</ref>." One more variation on solar stills is caught in the middle of the two predetermined categories of passive and active, and could perhaps be labeled "neutral". Seawater Greenhouses (i.e. [[Kiva's straw bale greenhouse]]) marry the concept of solar distillation with the more prominent greenhouse premise. They are neutral because the energy that goes in to create the freshwater, even if active, pays off by the freshwater's invaluable quality to grow the plants that promote the [[evaporative cooling]] of the air inside, which, ultimately carries the moisture<ref name ="Paton">Paton, C., & Davies, P. (2006). The seawater greenhouse cooling, fresh water and fresh produce from seawater. In The 2nd International Conference on Water Resources in Arid Environments, Riyadh.</ref>.[[File:Seawater_Greenhouse.jpg|thumb|upright=2|Seawater Greenhouse<ref name = "Paton"/>]]'''Active Solar Stills'''These distillers use additional heat sources to promote existing thermal processes<ref>AYBAR, H. (2007). A review of desalination by solar still. Solar Desalination for the 21 st Century, 207-214.</ref>. The foundation of the design of these desalters has already been lain in the above section, so the sources involved with this branch of solar stills will be discussed with brevity:*[[CPC]]s (Compound Parabolic [Solar] Concentrators)*Flat Plate Collectors<ref>Kabeel, A. E., & El-Agouz, S. A. (2011). Review of researches and developments on solar stills. Desalination, 276(1), 1-12.</ref>*Solar Heater<ref name = "Fath"/>*Novel Waste Heat (i.e. vehicle radiator)<ref>Mandaville, J. (1972). Some Experiments with Solar Ground Stills in Eastern Arabia. Geographical Journal, 64-66.</ref>Active stills add another element of complexity to the not so complex base design, but once again this alteration can promote faster, and larger quantities of freshwater generation.

==Theory==

The immediate abstraction to make is to the Earth's natural system, but as it was aforementioned, this is unjustified but only if one believes that the water cycle on Earth is a non-complex concept. In "Understanding Solar Stills" it is said,<blockquote>

"It takes a lot of energy for water to vaporize. While a certain amount of energy is needed to raise the temperature of a kilogram of water from 0 to 100 Celcius (C), it takes five and one-half times that much to chnage it from water at 100°C to water vapor at 100°C. Practically all this energy, however, is given back when the water vapor condenses... This is the way we get fresh water in the clouds from the oceans, by solar distillation. All the fresh water on earth has been solar distilled.<ref name = "Gordes"/>"

</blockquote>

The journey for a water molecule from the aqueous to gaseous phase is more difficult to acquiesce than the eloquent writing above. As we will see a tremendous factor will be the difference in temperature between the surface water and that of the interface, be it glass or plastic. Some relevant equations include<ref>

''Eqn. (1)'' from the above describes the instantaneous thermal efficiency in relation to the evaporative heat transfer rate from the water surface to the glass cover the solar radiation intensity.

''Eqn.(2)'' represents the evaporative heat transfer rate from ''Eqn. (1)'' and its relationship to product of the convective heat transfer coefficient from the water surface to glass difference between the partial vapor pressure of water and gas.

''Eqn.(3)'' is the equation for determining the monthly output of distillate.

''Eqn.(4)'' was developed to describe the pay back period, n<sub>p</sub> as a function of the Unacost, or the uniform end-of-year annual amount with P being the initial cost and i the interest rate.

==Construction==

Many different methods exist to build solar stills, the most rudimentary involves digging a hole and more the more complex coming off of a manufacturing line.

Common Materials List<ref name = "Gordes"/>:

*insulation (usually under the basin)

*sealants

*piping and valves

*facilities for storage

*a reflector to concentrate sunlight

*structural components

*wicking fabrics

*blackened wood

*black jute

*black polyethylene

Locally available materials would always be preferred but many things, such as the sealants, might be necessary to find from foreign vendors<ref name = "Velmu"/>. A very practical and thorough tutorial on how to build a small-scale solar still is available here:

Since one of the main goals of solar distillation is to provide a clean source of water, proper disinfection after construction is crucial. Some less intensive methods of cleaning could be use of soap or laundry detergent<ref name ="Gordes"/>. A glass cover is more advantageous with respect to maintenance than a plastic cover, due to the electrostatic properties of plastic that can make it a beacon for detritus<ref name=Modifications"Eibling"/>. The brine left over after thorough distillation can be harvested for sea salt, as it is now a valuable commodity in its own right <ref>KOPSCH, O. (2007). SOLAR STILLS: 10 YEARS OF PRACTICAL EXPERIENCE IN COMMERCIALISING SOLAR STILLS WORLDWIDE. Solar Desalination for the 21 st Century, 239-246.</ref>. ==Real World==In "Social aspects of a solar-powered desalination unit for remote Australian communities" Warner & Schafer argue that many journals, researchers and others like them rely too much on the technical aspects of solar distillation to prove its worth, "inorder to be socially sustainable, such technologies must:*be accepted by the community,*meet their water needs, and*be within their capacity to operate and maintain.<ref>Werner, M., & Schäfer, A. I. (2007). Social aspects of a solar-powered desalination unit for remote Australian communities. Desalination, 203(1), 375-393.</ref>" The situation that exists today has not changed much from even 50 years ago. Energy and cost intensive technologies still champion over desalination in the modern world<ref>Chaibi, M. T. (2000). An overview of solar desalination for domestic and agriculture water needs in remote arid areas. Desalination, 127(2), 119-133.</ref>. For this reason, many developing countries and communities, on large and small-scale levels resort to the status quo, when more appropriate resorts exist<ref>Bloemer, J. W., Eibling, J. A., Irwin, J. R., & Löf, G. O. (1965). A practical basin-type solar still. Solar Energy, 9(4), 197-200.</ref>.==Existing Products=='''[[Watercone]]'''

Cost has been a major barrier to implementing this. Recent work using [[CPC]]s has shown that solar distillation can be economically viable in some locations.

==[[Watercone]]==

The Watercone®[http://www.watercone.com] is a solar powered water desalinator. It is claimed to be simple to use, lightweight and mobile. It is designed to produce 1.5 liters a day.

Single products are not available at the moment![http://www.watercone.com]</blockquote>

'''Cost''': The planned price is below € 20,[http://www.watercone.com/product.html]. Solar distillation needs to become much cheaper than this before it can achieve widespread use by the poor. The website states that this works out cheaper than [[bottled water]] at 50c $.50 per liter once it is used for a number of months; however the target market cannot afford to buy bottled water, so this is not a useful comparison. If they do buy water, it is more likely to be from [[water refill stations]] which charge around 3 c $.03 per liter in major cities in Asia. In isolated areas, the costs increase a lot, but they would need to increase far beyond 3c $.03 per liter to justify the investment by a poor person or family - especially when it would be difficult to guard against theft. Thus it looks like they’re only useful where safe water is exceptionally expensive, or simply unavailable. Even then, other options for purifying the water would need to be weighed up. If these things were mass-produced for more like 1 euro or less each, they might be an option for widespread use - and this would be a more reasonable price for mass-produced pieces of molded plastic (even if they are very cleverly designed pieces of molded plastic).

===Watercone external links===

*[http://www.blog.thesietch.org/2007/08/13/watercone-ingenious-way-to-turn-salt-water-to-fresh-water/ Watercone - An Ingenious Way To Turn Salt Water Into Fresh Water] - ''The Sietch Blog'', August 2007